1. Technical Field
The present invention relates to a battery pack that is formed by connecting a plurality of secondary unit batteries, such as lithium ion batteries.
2. Background Art
A lithium ion secondary battery, in which charge and discharge take place as lithium ions move between a negative electrode and a positive electrode, has the following battery characteristics: high energy density and high output power. Therefore, in recent years, the lithium ion secondary battery has been used in various fields. For example, as an energy source for an electric power-assisted bicycle, a battery pack in which a plurality of secondary unit batteries, such as lithium ion batteries, is connected in series may be used.
For the exterior of a secondary unit battery that is used in the above-described manner, a laminate film casing material, which is made of a metallic laminate film, is used in many cases because of the following advantages: the laminate film casing material has a high degree of freedom in shape and is lightweight.
For example, what is disclosed in FIGS. 3 and 4 of Patent Document 1 (JP-A-2010-170799) is an assembled battery 23 in which a plurality of unit batteries 21, which are made from flat non-aqueous electrolyte batteries having a laminate film casing material, is stacked in such a way that negative terminals 6 and positive terminals 7, which extend out of the unit batteries 21, are arranged in the same direction, with an adhesive tape 22 binding the unit batteries 21 together. In the assembled battery 23, a plurality of unit batteries 21 is electrically connected in series to each other.
A battery pack disclosed in Patent Document 1 adopts the following structure: a plurality of unit batteries 21, which are stacked in such a way that the negative terminals 6 and the positive terminals 7 are arranged in the same direction, is electrically connected in series. In order for the unit batteries 21 to be connected in series, the electrodes of different polarities of adjoining unit batteries 21 need to be connected to each other. However, in the battery pack disclosed in Patent Document 1, electric connection sections of the adjoining unit batteries 21 are each disposed obliquely with respect to a direction in which the unit batteries 21 are stacked.
Moreover, as for the flat unit batteries 21 having a laminate film casing material as disclosed in Patent Document 1, the distance between electrodes of the adjoining unit batteries 21 is very short. For the above reason, in a process of producing the battery pack disclosed in Patent Document 1, attention needs to be paid to the following when the electrodes of adjoining unit batteries 21 are connected: a short-circuit between electrodes that are disposed in a small space at a time when the unit batteries 21 are sequentially connected obliquely with respect to the stacking direction. Therefore, the problem is that work efficiency is poor, and that productivity is low.